refactor: fix 3.12 syntax

Author: mtache

anta/result_manager/models.py

@@ -178,7 +178,9 @@ class TestResult(BaseTestResult):
     def __str__(self) -> str:
         """Return a human readable string of this TestResult."""
         results = f"{self.result} [{','.join([str(r.result) for r in self.atomic_results])}]" if self.atomic_results else str(self.result)
-        return f"Test {self.test} (on {self.name}): {results}{f'\nMessages:\n{"\n".join(self.messages)}' if self.messages else ''}"
+        lines = "\n".join(self.messages)
+        messages = f"\nMessages:\n{lines}" if self.messages else ""
+        return f"Test {self.test} (on {self.name}): {results}{messages}"
 
     def add(self, description: str | None = None, inputs: BaseModel | None = None) -> AtomicTestResult:
         """Create and add a new AtomicTestResult to this TestResult instance.